Wireless local area networks (WLANs) are gaining in popularity, and the demand for WLAN bandwidth is growing. The original WLAN standards, such as IEEE 802.11, were designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEE working groups have defined the 802.11a, 802.11b and 802.11g extensions to the original standard, in order to enable higher data rates. The 802.11a standard, for example, provides data rates up to 54 Mbps over short distances in a 5 GHz band, while 802.11b defines data rates up to 11 Mbps in the 2.4 GHz band. The newer 802.11g standard permits data rates up to 54 Mbps in the 2.4 GHz, and is also backward-compatible with legacy 802.11b products. In the context of the present patent application and in the claims, the term “802.11” is used to refer collectively to the original IEEE 802.11 standard and all its variants and extensions, unless specifically noted otherwise.
As WLAN use increases, so does the probability that two mobile stations in the same WLAN will attempt to transmit signals simultaneously on the same frequency channel. If a WLAN access point receives signals simultaneously from two sources of similar strength on the same frequency channel, it is generally unable to decipher either signal. Both stations must then retransmit their signals, leading to a waste of bandwidth and degradation of the communication rate. The 802.11 standard provides a mechanism for avoidance of such contention based on clear channel assessment (CCA), which requires a station to refrain from transmitting when it senses other transmissions on its frequency channel. In practice, however, this mechanism does not prevent multiple stations from beginning their transmissions at about the same time. Furthermore, there are cases in which the physical layout of the WLAN environment prevents one mobile station from sensing the signals transmitted by another mobile station on the same channel, but still permits signals from both mobile stations to be received simultaneously by a given access point.